I. Field of the Invention
The present invention relates generally to power steering systems, and more particularly to apparatus for improving the efficiency of electro-hydraulically energized vehicular power steering systems.
II. Description of the Prior Art
A variety of power steering systems are known for providing a powered assist to the steering of a motor vehicle. For example, electro-hydraulic power steering (EHPS) systems typically comprise an electrically driven pump which provides a continuous flow of a pressurized fluid to an open-center control valve having an input shaft. As a torque (representative of the applied steering torque) is applied to the input shaft of the valve, selected ones of a plurality of control orifices comprised in the valve progressively close, thereby impeding the flow of pressurized fluid through the valve and causing fluid pressure within the system to rise. Virtually all of the rising system pressure is then applied to a power cylinder in order to provide steering assist. Unfortunately, the efficiency of pressure matched systems such as this is inherently low, because the energy involved in generating the portion of the pressurized fluid passing through only the control valve, and not through the power cylinder, is lost. This undesirably increases the current draw needed for operation, and in practice has probably limited the usefulness of EHPS systems to mid-sized automobiles, at most.
Indeed, in general, the use of other, conventional power steering systems having a pump driven by the vehicle engine entails a cost in fuel efficiency typically amounting to between about 21/2 and 5 percent, usually on the lower end of this cost for relatively larger vehicles. EHPS systems are capable of obviating perhaps 90 percent of this loss in fuel efficiency because they are more nearly matched to the load encountered and because they provide a flow more closely matched to that needed, in comparison to the conventional power steering systems. This savings is appreciable and would, in theory, decrease the cost of operating the vehicle and/or permit the vehicle manufacturer to charge a greater price for the vehicle. In practice, this savings may appear to be somewhat offset by the appreciable maximum current drawn by EHPS systems during parking maneuvers. This last problem tends to limit the use of EHPS systems to smaller vehicles.
Also known in the art are full electric power steering (EPS) systems providing somewhat higher operating efficiency generally and somewhat reduced maximum current draw during parking maneuvers. Typically however, such EPS systems suffer from other practical limitations which also tend to restrict their use to smaller vehicles. Depending upon the form taken by any particular EPS system, these practical limitations include excessive pinion loading and power transmission efficiency problems for column mounted EPS systems and mechanical over-constraint and application related packaging issues with other EPS systems. In addition, because directional application of their assist torque occurs as a "control-by-wire" function, EPS systems must be protected from a condition known in the industry as "auto-steer" (e.g., an unintended steering event). EHPS systems, on the other hand, are free of this issue in that their directional application of their assist torque is determined by control valve means operatively driven by the driver via a steering wheel. Thus, energy is applied to an EHPS system in a non-directionally sensitive manner from unidirectionally driven pumping apparatus.
It would be highly advantageous to have EHPS systems that possess operating efficiencies and maximum current draw during parking maneuvers that are substantially equal to those of the most efficient EPS systems. It would be particularly advantageous to have an EHPS system which had a theoretical hydraulic operating efficiency approaching 100%. It would be additionally advantageous to have a regenerative EHPS system allowing substantial recovery of energy from the pressurized fluid in the system when the host vehicle exits a turn. Finally, it should go without saying that it would also be highly advantageous to have such a system which provided for safe operation of the vehicle during failure of the powered assist provided by the system.